support custom bull version

[foxling]

[codecov[bot]]

Codecov Report

Merging #9 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #9   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          14     14           
=====================================
  Hits           14     14

Impacted Files	Coverage Δ
lib/bull.js	100% <100%> (ø)	:arrow_up:

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